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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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 */

package java.util;

/**
 * The <code>Dictionary</code> class is the abstract parent of any
 * class, such as <code>Hashtable</code>, which maps keys to values.
 * Every key and every value is an object. In any one <tt>Dictionary</tt>
 * object, every key is associated with at most one value. Given a
 * <tt>Dictionary</tt> and a key, the associated element can be looked up.
 * Any non-<code>null</code> object can be used as a key and as a value.
 * <p>
 * As a rule, the <code>equals</code> method should be used by
 * implementations of this class to decide if two keys are the same.
 * <p>
 * <strong>NOTE: This class is obsolete.  New implementations should
 * implement the Map interface, rather than extending this class.</strong>
 *
 * @author unascribed
 * @see java.util.Map
 * @see java.lang.Object#equals(java.lang.Object)
 * @see java.lang.Object#hashCode()
 * @see java.util.Hashtable
 * @since JDK1.0
 */
public abstract class Dictionary<K, V> {

  /**
   * Sole constructor.  (For invocation by subclass constructors, typically
   * implicit.)
   */
  public Dictionary() {
  }

  /**
   * Returns the number of entries (distinct keys) in this dictionary.
   *
   * @return the number of keys in this dictionary.
   */
  abstract public int size();

  /**
   * Tests if this dictionary maps no keys to value. The general contract
   * for the <tt>isEmpty</tt> method is that the result is true if and only
   * if this dictionary contains no entries.
   *
   * @return <code>true</code> if this dictionary maps no keys to values; <code>false</code>
   * otherwise.
   */
  abstract public boolean isEmpty();

  /**
   * Returns an enumeration of the keys in this dictionary. The general
   * contract for the keys method is that an <tt>Enumeration</tt> object
   * is returned that will generate all the keys for which this dictionary
   * contains entries.
   *
   * @return an enumeration of the keys in this dictionary.
   * @see java.util.Dictionary#elements()
   * @see java.util.Enumeration
   */
  abstract public Enumeration<K> keys();

  /**
   * Returns an enumeration of the values in this dictionary. The general
   * contract for the <tt>elements</tt> method is that an
   * <tt>Enumeration</tt> is returned that will generate all the elements
   * contained in entries in this dictionary.
   *
   * @return an enumeration of the values in this dictionary.
   * @see java.util.Dictionary#keys()
   * @see java.util.Enumeration
   */
  abstract public Enumeration<V> elements();

  /**
   * Returns the value to which the key is mapped in this dictionary.
   * The general contract for the <tt>isEmpty</tt> method is that if this
   * dictionary contains an entry for the specified key, the associated
   * value is returned; otherwise, <tt>null</tt> is returned.
   *
   * @param key a key in this dictionary. <code>null</code> if the key is not mapped to any value in
   * this dictionary.
   * @return the value to which the key is mapped in this dictionary;
   * @throws NullPointerException if the <tt>key</tt> is <tt>null</tt>.
   * @see java.util.Dictionary#put(java.lang.Object, java.lang.Object)
   */
  abstract public V get(Object key);

  /**
   * Maps the specified <code>key</code> to the specified
   * <code>value</code> in this dictionary. Neither the key nor the
   * value can be <code>null</code>.
   * <p>
   * If this dictionary already contains an entry for the specified
   * <tt>key</tt>, the value already in this dictionary for that
   * <tt>key</tt> is returned, after modifying the entry to contain the
   * new element. <p>If this dictionary does not already have an entry
   * for the specified <tt>key</tt>, an entry is created for the
   * specified <tt>key</tt> and <tt>value</tt>, and <tt>null</tt> is
   * returned.
   * <p>
   * The <code>value</code> can be retrieved by calling the
   * <code>get</code> method with a <code>key</code> that is equal to
   * the original <code>key</code>.
   *
   * @param key the hashtable key.
   * @param value the value.
   * @return the previous value to which the <code>key</code> was mapped in this dictionary, or
   * <code>null</code> if the key did not have a previous mapping.
   * @throws NullPointerException if the <code>key</code> or <code>value</code> is
   * <code>null</code>.
   * @see java.lang.Object#equals(java.lang.Object)
   * @see java.util.Dictionary#get(java.lang.Object)
   */
  abstract public V put(K key, V value);

  /**
   * Removes the <code>key</code> (and its corresponding
   * <code>value</code>) from this dictionary. This method does nothing
   * if the <code>key</code> is not in this dictionary.
   *
   * @param key the key that needs to be removed.
   * @return the value to which the <code>key</code> had been mapped in this dictionary, or
   * <code>null</code> if the key did not have a mapping.
   * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
   */
  abstract public V remove(Object key);
}
